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Measurements from four case studies in spring and summer-time Arcticstratocumulus clouds during the Aerosol-Cloud Coupling And ClimateInteractions in the Arctic (ACCACIA) campaign are presented. We comparemicrophysics observations between cases and with previous measurements madein the Arctic and Antarctic. During ACCACIA, stratocumulus clouds wereobserved to consist of liquid at cloud tops, often at distinct temperatureinversions. The cloud top regions precipitated low concentrations of iceinto the cloud below. During the spring cases median ice numberconcentrations (~ 0.5 L−1) were found to be lower byabout a factor of 5 than observations from the summer campaign(~ 3 L−1). Cloud layers in the summer spanned a warmertemperature regime than in the spring and enhancement of ice concentrationsin these cases was found to be due to secondary ice production through theHallett–Mossop (H–M) process. Aerosol concentrations during spring rangedfrom ~ 300–400 cm−3 in one case to lower values of~ 50–100 cm−3 in the other. The concentration of aerosolwith sizes Dp > 0.5 μm was used in a primary icenucleus (IN) prediction scheme (DeMott et al., 2010). Predicted IN valuesvaried depending on aerosol measurement periods but were generally greaterthan maximum observed median values of ice crystal concentrations in thespring cases, and less than the observed ice concentrations in the summerdue to the influence of secondary ice production. Comparison with recentcloud observations in the Antarctic summer (Grosvenor et al., 2012), revealslower ice concentrations in Antarctic clouds in comparable seasons. Anenhancement of ice crystal number concentrations (when compared withpredicted IN numbers) was also found in Antarctic stratocumulus cloudsspanning the H–M temperature zone; however, concentrations wereabout an order of magnitude lower than those observed in the Arctic summercases but were similar to the peak values observed in the colder Arcticspring cases, where the H–M mechanism did not operate

Observations and comparisons of cloud microphysical properties in spring and summertime Arctic stratocumulus clouds during the ACCACIA campaign